NEBB ETS Chapter 11 Cards

Space Air Distribution

What is the acceptable effective draft temperature?

It includes air temperature, air motion, relative humidity, and physiological effects on the human body.

What air velocity range is recommended for comfort in the occupied zone?

20 fpm to 70 fpm, with 50 fpm being the norm.

What is the process of air entrainment?

It is the induction of secondary air movement into the primary air stream to create total air movement.

What is the Coanda effect in air distribution?

It describes how a jet of air attaches to a surface, affecting air flow and distribution.

How does the surface effect influence ceiling diffusers?

It allows a circular air pattern to blanket the ceiling area surrounding each outlet.

What is smudging in relation to air distribution systems?

It is discoloration caused by dirt particles in turbulent air at the outlet face.

How do natural convection currents affect air distribution?

They cause stratification of air layers, impacting temperature distribution in perimeter zones.

What is the recommended location for supply air outlets to combat stratification?

High in the wall or in the ceiling.

What happens when the blade ratio of a grille is less than one?

Effective control of the airstream discharged from the grille is impossible.

How does increasing blade divergence affect air discharge?

It reduces the throw and quantity of air discharged by the grille.

What is the impact of high pedestrian traffic on air distribution systems?

It can lead to smudging around diffusers due to turbulence created by human movement.

What temperature differential range should be maintained in air conditioning systems?

15°F to 20°F.

How large is the critical area of the occupied zone in HVAC systems?

It is the area from the floor to 6 feet above floor level.

What is the role of primary air in air distribution?

It induces movement of secondary air within the room.

What is the effect of high velocities at ankle level compared to neck level?

People tolerate higher velocities and lower temperatures at ankle level better than at neck level.

What is the relationship between outlet velocity and sound levels?

Higher discharge velocities can lead to higher frequency sounds from the outlet.

What happens to the air when it is discharged from a supply outlet?

It induces air movement and can create a low-pressure bubble that stabilizes the airflow.

What type of blades does a supply air grille with converging blades have?

Vertical blades with uniformly decreasing angular deflection from the centerline.

What happens to the airstream in front of a supply air outlet?

It converges slightly for a short distance and then spreads more rapidly.

What is a potential risk of using horizontal blades in air distribution?

It may hit beams or other obstructions or blow air at excessive velocities into the occupied zone.

Why is vertical deflection of air beneficial?

It may increase adherence to the ceiling and reduce the drop of the air stream.

Where should outlets be located in spaces with exposed beams?

Below the bottom of the lowest beam level, preferably low enough to employ an upward or arched air path.

What is the minimum outlet discharge velocity to prevent cool air from dropping back into the occupied space?

500 fpm (2.5 m/s) minimum.

What are the five groups of supply air outlets?

Group A: Ceiling outlets discharging horizontally; Group B: Floor outlets discharging vertically in a nonspreading jet; Group C: Floor outlets discharging vertically in a spreading jet; Group D: Floor outlets discharging horizontally; Group E: Ceiling outlets projecting primary air vertically.

What is the relationship between initial velocity and throw in air distribution?

Doubling the initial velocity for the same terminal velocity doubles the induction ratio and also the throw.

How does total air behave during heating and cooling?

It drops during cooling and rises during heating, influenced by the primary air and environment.

What is a stagnant zone in air distribution?

A zone formed by natural convection currents where air velocities are usually low, around 20 fpm.

What is the role of a return inlet in air distribution?

It should be located in the stagnant zone to return the warmest air during cooling or the coolest air during heating as it affects the movement of air only in the immediate vicinity of the inlet.

What is the significance of the throw coefficient in variable air volume applications?

A low throw coefficient minimizes changes in air motion within the occupied space due to variations in airflow as the air supplied is meant to provide comfort only through thermal changes and not by air motion, hence why a smaller throw is acceptable in a VAV application.

What is the recommended outlet type for under window distribution?

Vertical throw outlets with a nonspreading pattern.

What should be considered when analyzing outlet performance?

Primary air pattern, total air pattern, stagnant air layer, natural convection currents, return air pattern, and room air motion.

What is the effect of temperature variations within a room with ceiling outlets?

They minimize temperature variations and stagnant volume.

What is the primary air envelope in air distribution with regards to sidewall and ceiling diffusers?

It shows the horizontal, two-jet pattern produced by high sidewall outlets and a 360° diffusion pattern produced by ceiling outlets.

What factors influence the distance at which total air drops into the occupied zone?

Air quantity, supply velocity, temperature differential, deflection setting, ceiling effect, and type of loading.

What is the significance of the drop region in air distribution?

The drop region is important because it has high effective draft temperature, where maximum velocity and temperature variation occur.

How does the positioning of outlets affect air mixing?

The outlet position will dictate where air mixing and drop regions occurs.

What can cause stratification in a room during heating?

Warm supply air introduced at the ceiling can cause stratification if there is NOT sufficient induction of room air at the outlet.

What factors are important for adequate air diffusion?

Proper diffuser selection, limiting room supply temperature differential, and maintaining high enough air supply rates to ensure adequate air diffusion.

How do Group A outlets differ from Group B outlets?

Group A outlets have deflecting vanes and create a larger stagnant zone, while Group B outlets discharge primary air in a single vertical jet, resulting in a smaller stagnant region.

What is the effect of high velocity in air distribution?

Higher velocities can eliminate stagnant zones and help warm air reach floor level, counteracting stratification.

What characterizes Group C outlets?

Group C outlets have wide spreading jets and diffusing action, with a larger stagnant zone during cooling and a smaller one during heating.

What happens to total air during cooling with Group D outlets?

Total air remains near the floor, creating a large stagnant zone in the upper region of the room.

How do Group E outlets function during heating and cooling?

During cooling, total air projects to the floor, creating a stagnant region near the ceiling; during heating, it folds back toward the ceiling.

What factors influence the selection of supply outlets?

Air quantity required, distance for throw or diffusion radius, structural characteristics, and architectural concepts are considered.

What is the difference between a grille and a register?

A grille has fixed blades while a register includes adjustable volume control blades.

What is a single deflection grille?

A single deflection grille consists of a frame enclosing vertical or horizontal blades that deflect the airstream in one plane.

What is the purpose of a double deflection grille?

A double deflection grille controls the airstream in both horizontal and vertical planes with a second set of blades.

What is a stamped grille and what is its purpose?

A stamped grille is made from a single sheet of metal, forming a fretwork design through which air can pass. They are used as return air and exhaust air inlets, particularly in restrooms and utility areas.

What is a variable area grille?

A variable area grille can change the discharge area to achieve air volume changes at constant pressure, minimizing throw variation.

What is the effect of induction of room air at outlets and how does initial velocity affect it?

Induction of room air at outlets enhances air mixing and improves temperature distribution in the space. Higher throw velocities = higher induction rates.

What is the impact of ceiling height on air distribution?

Ceiling heights greater than normal can modify outlet performance and require adjustments in air distribution design.

What is the significance of the terminal point in air distribution?

The terminal point is where warm total air loses downward momentum and rises, impacting the cooling and heating dynamics.

What aspect ratio is generally required for slot diffusers to meet performance criteria?

A dimensional aspect ratio of 25 to 1 or greater with a maximum height of 3 inches.

What are air-light slot diffusers designed for?

They are designed for use with recessed fluorescent light troffers.

What are multi-passage ceiling diffusers composed of?

A series of flaring rings or louvers that form concentric air passages.

What is the difference between flush and stepped-down diffusers?

Flush units project a plane surface, while stepped-down units project beyond the surface.

What is the function of deflection devices in perforated face ceiling diffusers?

To obtain multi-pattern horizontal air discharge.

What is the purpose of variable area ceiling diffusers?

To vary the discharge area for air volume change at constant pressure.

What is crucial for correct air diffusion at outlets?

The velocity of the airstream must be uniform and perpendicular to the outlet face.

What is the recommended air velocity for return air inlets to minimize noise?

Should not exceed approximately 500 fpm (2.5 m/s).

What is the typical free area percentage for commercial return grilles?

Between 45 and 55%.

What should be avoided when locating return air inlets?

They should not be located directly in the primary airstream from supply outlets.

What is the effect of stratifying air before mixing in a space?

It permits lower temperature differentials or lower airflow rates.

What is the upper limit and lower limit of plenum pressure for air distribution ceilings?

The maximum generally ranges from 0.10 to 0.15 in.w.g. (25 to 38 Pa) while the lower limit is about 0.01 in.w.g.

What is the recommended air rate for air distribution ceilings?

From about 15 cfm (7.5 L/s) to about 1 cfm per square foot of floor area.

What is the impact of improper duct approaches on air outlets?

No air outlet can compensate for airflow from an improper duct approach.

What is the recommended installation for sidewall outlets?

They should be installed at the end of vertical ducts or in the side of horizontal ducts.

What are the benefits of using multiple curved deflectors in ducts?

They produce the best flow characteristics.

What is a key consideration when sizing return air inlets?

They should be sized based on available pressure requirements and sound data.

What type of grilles can be adjusted to match supply outlets?

Adjustable Blade Grilles

What is the most common type of return air inlet grille?

Fixed Blade Grilles

What is a V-Blade Grille designed for?

To be sightproof and maintain appearance from any angle.

What type of grille is used in darkrooms?

Lightproof Grilles

What is displacement ventilation?

A system that supplies conditioned air at low velocities from floor level, allowing warm air to be exhausted from the ceiling (floor level supply air displaces the warm up into the ceiling returns/exhausts) creating a two layers of stratification. This flow of air also helps entrain surrounding ambient air for even better ventilation due to natural convection forces.

What is the maximum convective cooling load recommended for office buildings using displacement ventilation?

About 8 Btu/h x ft² to maintain a vertical temperature gradient of no more than 5°F.

How does localized ventilation differ from conventional ceiling-based air diffusion?

Localized ventilation supplies air directly to the occupied zone with higher supply volumes and velocities.

What is the purpose of task conditioning systems?

To allow individuals control over their local environment by adjusting supply air volume and direction.

What is unidirectional airflow ventilation used for?

To provide low turbulence airflow primarily in cleanrooms and areas requiring contaminant removal.

What defines building pressurization?

The static pressure difference between the interior and exterior pressure of a building.

What is the recommended positive pressure for buildings for HVAC system designers?

A slight positive pressure of about 0.05 in.w.g. to prevent infiltration of contaminants, dictating a return/exhaust ratio of 80-90% of total supply air.

How can the cooling capacity of displacement ventilation systems be increased?

By recirculating some room air through an induction circuit (mixing with 50 degree primary air) before discharging it at a more acceptable temperature (61-68F). This effectively becomes a two-stage cooling system--cooling within the induction circuit and cooling again when supplied into the room.

What are the typical supply air temperatures for localized ventilation systems?

Usually above 63°F to avoid local draft discomfort.

What is the purpose of negative pressure in a conditioned space?

To prioritize the management of odors, contamination, or prevent exfiltration.

What are the three types of supply air systems for building pressurization?

1. Single-fan system; 2. Supply and return air fan system with constant volume return fan; 3. Supply and return air fan system with modulation on both fans.

What is the general challenge of single fan systems (even more so when using outside air economizers)? What is the recommended solution?

They can create overpressurization, making temperature control difficult. Installing a relief air exhaust fan can help alleviate the concern as it will manage excess pressure and maintain balance in the system.

How does a two fan system perform with constant volume return air fan?

It maintains acceptable pressurization regardless of economizer mode.

What happens when the supply air fan volume is less than the return air fan volume?

The return air fan attempts to remove more air than is delivered, creating negative pressure.

What is the effect of VAV boxes being throttled back in the morning?

It creates the greatest negative pressure in buildings.

What are the two methods of return air fan modulation control?

1. Single static pressure probe in the supply air duct; 2. Interactive controller using the airflow of the supply and return fans.

What is the primary advantage of using exhaust air fans in VAV systems when in 100% economizer mode?

Simpler and less expensive pressurization control.

What is the pressure drop across barometric relief dampers and louvers?

Typically 0.25 in.w.g. or greater.

What is the impact of outside air damper leakage in single fan systems?

It is higher due to the negative static pressure developed by the supply fan. Less impact when a two-fan system is used.

What is the secondary role of economizers in HVAC systems?

To replace exhaust air with outside air, maintaining indoor air quality and building pressure.

What is the consequence of not using modulation in return air fans?

It can lead to large pressure variations in the building.

What is the primary control loop and efficiency difference between utilizing an exhaust air versus a return air fan system?

Exhaust air fans have simpler independent control loops and are more efficient (only run when needed) than return fan implementations within a unit.

What is the main disadvantage of barometric relief dampers?

They may become inoperative due to pressure differences.

What is the advantage of using airflow measuring stations in HVAC systems? What is the disadvantage?

To achieve better control of building pressurization. They add significant complexity and cost to the return air fan system.

What is the advantage of using a single fan system on return airflow?

It handles all return system losses, making it ideal for shutoff VAV systems.

What is the typical static pressure range developed by a supply fan across outside air dampers?

-0.3 to -1.0 in.w.g.

What signals are used to position the airflow device?

The building pressurization signal and a supply airflow rate signal.